Planographic printing plate assembly and method of making

ABSTRACT

Thermographic copying process employing planographic printing plate assembly comprising a translucent film-base printing plate and a translucent film-base transfer sheet containing a heattransferable oleophilic imaging layer. The plate comprises a normally heat-shrinkable plastic film foundation having a thin planographic layer thereon and imaging occurs at a temperature below the shrink temperature of the plate.

States atent Newman 1 1 June 5, 1973 54] PLANOGRAPHIC PRINTING PLATE 3,364,858 l/l968 Kovimn ..101/467 ASSEMBLY AND METHOD 3,588,501 6/1971 Lowrey ..250/65T MAKING [75] Inventor: gopglas A. Newman, Glen Cove, Primary Examiner Murray Katz Attorney-Johnson&Kline [73] Assignee: Columbia Ribbon and Carbon Manufacturing Co., Inc., Glen Cove, 57 ABSTRACT [22] Filed: July 12,1971 Thermographic copying process employing plano- [21] Appl. No.: 161,743

[52] US. Cl. ..101/467, l17/36.l, ll7/l38.8 UA, 252/65 [51] Int. Cl. ..B4lm l/06, B4ln 1/12 [58] Field of Search ..l l7/36.1; l0l/467, l0l/470; 250/65.l

[56] References Cited UNITED STATES PATENTS 3,055,295 9/1962 Pericins ..101/462 graphic printing plate assembly comprising a translucent film-base printing plate and a translucent filmbase transfer sheet containing a heat-transferable oleophilic imaging layer. The plate comprises a normally heat-shrinkable plastic film foundation having a thin planographic layer thereon and imaging occurs at a temperature below the shrink temperature of the plate.

10 Claims, 2 Drawing Figures PATENTEDJUH 5 ma 3,736,873

INVENTOR.

BY W M kQq flTT'OEMEYS PLANOGRAPHIC PRINTING PLATE ASSEMBLY AND METHOD OF MAKING Thermographic planographic printing plate units and assemblies have been proposed but none of these have met with widespread commercial use or success. Proposed assemblies comprise a semi-translucent film-base plate and a translucent oleophilic transfer sheet and are exemplified by British Pat. No. 958,485. Such film-base plates have a film foundation such as Mylar polyethylene terephthalate polyester, cellulose acetate, or the like, carrying a conventional thin planographic printing layer containing a hydrophilic binder material and an opaque filler. The transfer sheet carries an oleophilic wax imaging layer which is heat meltable in the thermographic transfer process and may be tinted for proofreading purposes.

Such assemblies have been found unacceptable for quality copying purposes because they do not consistently provide a good copy of the imaged original sheet in the reflex thermographic process. The reflex exposure process is necessary for copying most original sheets since such sheets are semi-opaque and/or have images on both surfaces thereof. Such originals must be positioned furthest from the radiation source and the plate assembly must be positioned between the original and the radiation source.

Prior assemblies do not function well in the reflex exposure process. They have a very narrow functional temperature range, below which they provide partial or spotty imaging of the plate surface and above which they provide excess melting, broadening and fill-in of the images formed on the plate surface. The selection of the correct temperature is difficult because the exposure machine builds up ambient heat during repeated use so that the speed at which the assembly is passed through the machine must be increased gradually in order to attempt to insure a uniform exposure temperature when a number of successive assemblies are being imaged.

Prior film-base thermographic printing plates also have serious curling disadvantages which render them difficult to pass through a thermal copying machine and difficult to mount and run on a planographic printing machine.

It is the principal object of the present invention to provide thermographic planographic assemblies which have a wide functional temperature range whereby they consistently produce sharp and clear images on the plate surface, approaching the exact quality of the original images, when exposed at a wide range of temperatures and speeds in thermographic copying machines.

It is another object of this invention to provide coated thermographic planographic printing plates which are translucent and even transparent and yet have excellent receptivity for heat-transferred images and excellent retentivity of such images during the planographic printing process.

It is still another object of one embodiment of this invention to provide improved coated planographic printing plates which have little or no tendency to curl and thus are dimensionally stable under conditions of imaging and duplicating.

These and other objects of this invention will be apparent to those skilled in the art in the light of thepresent disclosure, including the drawing in which:

FIG. 1 is a diagrammatic cross-section, to an enlarged scale, of the reflex thermographic process for imaging the assemblies of the present invention.

FIG. 2 is a diagrammatic cross-section, to an enlarged scale, of a planographic printing plate imaged according to FIG. 1 and suitable for dual use as both a projection transparency and a printing plate.

The present invention involves the discovery that heat-shrinkable films, heretofore considered to be unsatisfactory coating supports in the thermographic transfer process, provide unexpectedly good results when used as film foundations for certain planographic printing coatings in such a process. Such films have good retention properties for such planographic printing layers and/or for intermediate layers carrying such printing layers, particularly under the effects of heat. This appears to be due to the fact that heat-shrinkable films, being oriented but not heat-set, have surface properties and physical properties which are different than unoriented films or oriented, heat-set films and which are especially advantageous in the thermographic process.

In view of these properties, surface adhesion between the plate foundation and the coating thereon is such that the filler material, normally essential to planographic coatings, can be completely omitted provided that the planographic composition is one based upon a major amount by weight of a colloidal silica having an average particle size of from about 40 to millimicrons. Since conventional fillers, of which clay is the most commonly used, are opaque, the omission of such fillers makes it possible to provide planographic coatings which are either transparent or are translucent to a higher degree than prior known thermographic planographic layers. Such prior known layers always included a higher amount of opaque filler and represented a compromise between good adhesion to the film foundation and high translucency. The enlarged silica particles assume the function of a filler to some extent by rendering the planographic layer surface rougher and more open and receptive, and also are found to provide a curl-resistant plate having high translucency.

The transparency or high translucence of the present plates makes them well suited for use in the reflex exposure process and creates greater latitude in the time and temperature of exposure to infrared radiation. The only limitations presented by such plates and the thermographic transfer sheets used therewith are that exposure must be sufficient to cause the transfer layer to soften or melt and adhere to the plate surface in the heated areas but insufficient to cause the heatshrinkable film to shrink or contract in the heated areas.

The preferred planographic composition for use according to the present invention is a hydrophilic binder-colloidal silica composition of the type disclosed in US. Pat. No. 3,055,295 with the exception that colloidal silica having an average particle size greater than 40 millimicrons is used and the filler is excluded. Small amounts of clay or other filler can be included but this is neither necessary nor desirable, the maximum permissible amount being equivalent to the dry weight of the hydrophilic binder material. The preferred binder is polyvinyl alcohol and the preferred filler is clay.

In general, conventional thermographic transfer sheets are suitable for use according to this invention provided that the transfer composition is oleophilic and compositions are hot-melt compositions based upon The unique planographic printing plates of the present invention comprise a clear heat-contractile plastic film having a thickness of up to about 3 mils. The preferred film is plasticized polystyrene having a thickness waxy binder materials and contain a small amount of -5 of 1 mil and which is coated with a very thin base layer dissolved dyestuff for proofreading or projection purposes.

Referring to FIG. 1 of the drawing, the present assembly comprising the film-base plate 20 and the thermal transfer sheet 30 is positioned over an imaged original sheet and exposed to a radiation source 40 rich in infrared radiation. This is the position of the sheets in the reflex exposure process, the sheets being shown out of contact for purposes of illustration.

The original sheet comprises a foundation 11 such as paper carrying infrared radiation-absorbing images 12 facing the radiation source 40. The plate 20 comprises a heat-contractile film foundation 21 such as polystyrene and a thin planographic printing layer 22 having the conventional hydrophilic-oleophilic balance. The transfer sheet 30 comprises a thin film foundation 31 such as Mylar carrying a thin heattransferable oleophilic layer 32.

A brief exposure to infrared radiation causes the images 12 to become heated and to generate an imagewise heat pattern which is conducted back to the oleophilic layer 32 to soften corresponding portions of that layer and cause them to adhere to the surface of layer 22. After exposure, the sheets are separated and the portions of layer 32 adhered to layer 22 remain bonded thereto in the form of oleophilic images 23 shown in FIG. 2.

The adhesion between contractile film 21 and layer 22 is sufficient to permit layer 22 to strip the imaging material from the transfer sheet. Also the adhesion between planographic layer 22 and oleophilic images 23 is sufficient for this purpose and also sufficient to retain the images 23 when the imaged plate is used in the planographic printing process. In such process, the imaged plate is wetted with aqueous etching fluid which is retained only in the unimaged surface areas of layer 23. Then the plate is inked with oleous ink which is retained only on the surface of oleophilic images 23 and is repelled from the other areas. The ink is then transferred to a roller and retransferred to a copy sheet to produce a duplicate copy of the plate. This is repeated with a succession of copy sheets to produce as many copies as desired.

According to a preferred embodiment, the printing plate assemblies of the present invention also comprise a support sheet attached along one edge to the rear surface of the filmbase plate. The support sheet functions to render the assembly more rigid and thus easier to handle and to introduce into the thermal copying machine and renders the imaged plate easier to attach to the planographic printing machine. Preferred support sheets are heavyweight paper in the order of 20 to 30 pounds per ream (3,300 square feet). The support sheet is attached to the rear of the plate along one edge and an imaged original is copied in the reflex procedure by inserting it between the support sheet and the rear surface of the plate with the original images positioned against the latter. After thermal exposure, the imaged original sheet is removed, the thermal transfer sheet is separated from the imaged plate, and the combination of the support sheet and the imaged plate is attached to a planographic printing machine for duplication.

of a hydrophilic film-forming material such as polyvinyl alcohol to render it more hydrophilic and receptive to the planographic coating composition. This base layer may be applied as a dilute solution of polyvinyl alcohol in water or a mixture of water and ethanol and dried by evaporation of the solvent, as taught for instance by US. Pat. No. 3,146,883.

Other suitable heat-contractile films include polyvinylidene chloride, polyethylene, polypropylene, styrene-acrylonitrile, and the like. It is preferred to pretreat the film to render it more hydrophilic, such as by corona discharge, coating, or the like.

The planographic coating composition comprises about 1 part by weight of a hydrophilic colloid binder material, preferably polyvinyl alcohol, from 2 to 5 parts by weight of colloidal silica having a particle size of from 40 to millimicrons, and from 0 to 1 part by weight of porous filler such as clay. Planographic layers of such composition which have a weight of from about 1 to 4 pounds per ream (3,300 square feet) are transparent or highly translucent due to the absence or low content of filler. The omission or restriction of filler content is made possible by the use of the enlarged silica particles which tend to render the planographic surface rougher and more open than corresponding planographic layers based upon the more conventional colloidal silica particles having an average particle size of from 7 to 30 millimicrons. These are functions heretofore performed by the porous filler to give the planographic layer good ink-drying properties and to render the layer receptive to and retentive of heat-transferred and pressure-applied images, the latter being applied in the case of preprinted plates for forms work. Thus the enlarged silica particles make it possible to omit the porous filler or at least to substantially reduce the amount of such filler to the point that its opaque properties do not interfere with the translucency and infrared radiation-transmitting properties of the planographic layer. The present printing plates also have little or no tendency to curl, due to the larger size of the silica particles and/or to the omission of large amounts of porous fillers such as clay.

The following example is given as an illustration of the manufacture and use of a thermal plate unit according to one embodiment of the present invention.

The thermal plate is produced in the following manner. A 1.0 mil film of plasticized polystyrene having a heat-contraction temperature of about C is coated on one surface with a base layer of percent hydrolyzed polyvinyl alcohol applied as a very thin wash comprising a 30 percent solution in a 3:4 mixture of water and ethanol. The solvents are evaporated to leave a nearly imperceptible thin hydrophilic polyvinyl alcohol base layer on the polystyrene film.

Next the following planographic composition is applied over the hydrophilic base layer as a uniform continuous coating.

Ingredients Parts by Weight Polyvinyl alcohol (8% aqueous) 8 Water 92 Colloidal silica (50% aqueous) 25 Water 25 Glyoxal (40% aqueous) 0.8 Water 1.2 Acetic acid 2 Butanol 3 The polyvinyl alcohol solution used is available under the trademark Elvanol 71-30 and is believed to be 98 percent hydrolyzed. The colloidal silica dispersion used is available under the trademark Nalcoag 1060 and has an average particle size of from 40 to 60 millimicrons. The glyoxal and the silica function as insolubilizing agents for the polyvinyl alcohol.

The planographic coating is dried and insolubilized by evaporation of the water and butanol to form a planographic printing layer having a weight of about 1 to 2 pounds per ream (3,300 square feet of film). The printing layer has very high translucency approaching transparency and the printing plate lies flat and has no tendency to curl under ambient conditions.

The thermal transfer sheet for use in association with the printing plate is produced in the following manner. A 0.5 mil film of polyethylene terephthalate polyester is coated on one surface with from 1 to 3 points of the following hot-melt composition (a point equals 0.0001 inch).

Ingredients Parts by Weight Camauba wax 50 The heat-sensitive transfer layer is oleophilic and is heat-transferable to the planographic printing layer at a temperature of from 60 C to 80 C, i.e., well below the contraction temperature of the polystyrene film foundation of the plate.

Referring to FIG. 1 of the drawing, the plate 20 and transfer sheet 30 are assembled with transfer layer 32 facing printing layer 22. The sheets may be attached along one edge, such as by means of a weak adhesive line, to form a unit. The unit may also contain a semirigid backing sheet attached along one edge to the rear surface of the plate, as discussed hereinbefore.

Variations and modifications may be made within the scope of the claims and portions of the improvements may be used without others.

I claim:

1. A thermographic planographic printing plate assembly comprising a transfer sheet having a plastic film foundation carrying a layer of heat-transferable oleophilic imaging composition and a planographic printing plate comprising a plastic film foundation carrying a translucent planographic printing layer characterized by the plastic film foundation of the plate being heatcontractile at a temperature above the heat-transfer temperature of the oleophilic imaging composition and said planographic printing layer comprising 1 part by weight of a hydrophilic colloid binder material, an insolubilizing agent therefor and from 2 to 5 parts by weight of colloidal silica having an average particle size between about 40 and millimicrons.

2. An assembly according to claim 1 in which the planographic printing layer composition also contains up to 1 part by weight of a porous filler.

3. An assembly according to claim 1 in which the planographic printing layer composition comprises polyvinyl alcohol as the binder material.

4. An assembly according to claim 1 in which the heat-contractile film foundation has a thin hydrophilic binder layer thereon to which the printing layer is applied.

5. An assembly according to claim 1 comprising a support sheet attached to the rear side of said heatcontractile film foundation.

6. An assembly according to claim 1 in which the heat-contractile film foundation comprises polystyrene.

7. Reflex process for making a copy of an imaged original sheet on a planographic printing plate by means of infrared radiation which comprises positioning the original sheet beneath and against an assembly of a translucent film-base planographic printing plate and a film-base transfer sheet carrying a heattransferable oleophilic layer, and applying infrared radiation through the assembly to heat the original images and cause the heat-transfer of corresponding areas of the oleophilic layer to the printing plate characterized by the printing plate comprising a heat-contractile film foundation carrying a planographic printing layer comprising 1 part by weight of a hydrophilic colloid binder material, an insolubilizing agent therefor and from 2 to 5 parts by weight of colloidal silica having an average particle size between about 40 and 70 millimicrons, and further characterized by heat-transfer of the oleophilic layer occurring at a temperature below the heat-contraction temperature of the plate foundation whereby contraction does not occur.

8. Process according to claim 7 in which the heatcontractile film has a contraction temperature within the range of from about C to 120 C.

9. Process according to claim 7 in which the heatcontractile film comprises polystyrene.

10. Process according to claim 7 in which the oleophilic layer comprises a hot-melt waxy composition having a heat-transfer temperature below about C. 

2. An assembly according to claim 1 in which the planographic printing layer composition also contains up to 1 part by weight of a porous filler.
 3. An assembly according to claim 1 in which the planographic printing layer composition comprises polyvinyl alcohol as the binder material.
 4. An assembly according to claim 1 in which the heat-contractile film foundation has a thin hydrophilic binder layer thereon to which the printing layer is applied.
 5. An assembly according to claim 1 comprising a support sheet attached to the rear side of said heat-contractile film foundation.
 6. An assembly according to claim 1 in which the heat-contractile film foundation comprises polystyrene.
 7. Reflex process for making a copy of an imaged original sheet on a planographic printing plate by means of infrared radiation which comprises positioning the original sheet beneath and against an assembly of a translucent film-base planographic printing plate and a film-base transfer sheet carrying a heat-transferable oleophilic layer, and applying infrared radiation through the assembly to heat the original images and cause the heat-transfer of corresponding areas of the oleophilic layer to the printing plate characterized by the printing plate comprising a heat-contractile film foundation carrying a planographic printing layer comprising 1 part by weight of a hydrophilic colloid binder material, an insolubilizing agent therefor and from 2 to 5 parts by weight of colloidal silica having an average particle size between about 40 and 70 millimicrons, and further characterized by heat-transfer of the oleophilic layer occurring at a temperature below the heat-contraction temperature of the plate foundation whereby contraction does not occur.
 8. Process according to claim 7 in which the heat-contractile film has a contraction temperature within the range of from about 80* C to 120* C.
 9. Process according to claim 7 in which the heat-contractile film comprises polystyrene.
 10. Process according to claim 7 in which the oleophilic layer comprises a hot-melt waxy composition having a heat-transfer temperature below about 90* C. 